Joplin, Missouri (CNN) -- The tornado that struck Joplin, Missouri, Sunday killed 118 people, authorities said Tuesday, making it the deadliest single U.S. tornado since modern record-keeping began more than 60 years ago.

Missouri Gov. Jay Nixon said Tuesday morning the death toll has risen to 118 and the number of deaths is expected to rise as rescuers find more bodies in the rubble.

A twister in Flint, Michigan, in 1953 killed 116 people, according to the National Weather Service.

2011 set to be deadliest tornado year

"We are going to do absolutely everything we can to make sure they recover," said President Barack Obama, who announced on Tuesday that he plans to visit the region this Sunday.

Obama said he will let people know "the whole country is going to be behind them."

Richard Serino, the second-highest ranking official at the Federal Emergency Management Agency, said Obama has issued a disaster declaration -- expediting the dispersal of federal resources to the area -- while vowing that "we are going to be here for the long haul."

Joplin may not be in the clear yet as far as weather goes: the National Weather Service warned there is a 45% chance of another tornado outbreak -- with the peak time between 4 p.m. and midnight Tuesday -- over a wide swath including parts of Texas, Oklahoma, Arkansas, Nebraska and Missouri.

Officials in Joplin announced plans to test their tornado sirens at 10:30 a.m. local time (11:30 a.m. ET).

"There's no way to figure out how to pick up the pieces as is," Sarah Hale, a lifelong Joplin resident, said Tuesday. "We have to have faith the weather will change."

"One, fortunately, walked away from it; the other one's still in the hospital, last I heard," Joplin Emergency Management Director Keith Stammer told CNN's "American Morning."

Nixon said the one in the hospital is in the intensive care unit.

Several hundred people were injured in the tornado, and about 1,500 people are still unaccounted for, Stammer said. "What that means is they've scattered," he said. "When we open up the area and starting letting them come back in... that number of unaccounted for will start to dwindle."

People who have left the area should call their families and the dispatch center to let authorities know they are OK, Stammer said.

"We are hoping that by the time the sun goes down tonight, we'll be done with our primary and our secondary search and rescue effort," he added.

Joplin Fire Chief Mitch Randles said the second search basically follows the tornado's path. "We're searching every structure that's been damaged or destroyed in a more in-depth manner," he said. "The third search is going to be similar to that. And then the fourth search through will be with the search and rescue dogs."

Authorities encouraged people to use the website safeandwell.org, operated by the Red Cross, for updates on loved ones.

Some residents said the tornado struck suddenly.

"It all happened so fast," Rachael Neff told "American Morning" Tuesday. "It seemed like forever but it happened very fast."

"We had a few minutes warning. I've never taken any of the warnings seriously but something snapped in me and I put blankets and pillows in the bathroom. We were running to the bathroom. You could hear the home shaking, everything busting out."

By Monday night, officials found 17 people alive. But many, including Will Norton, remain missing.

The 18-year-old was driving home from his high school graduation Sunday when the tornado destroyed the Hummer H3 he and his father were in.

"We were in a separate car. We were about 30 seconds in front of them, one block," Norton's sister, Sara, told CNN. "My dad called and he said, 'Open the garage door.' ... And then I just heard him say, 'Pull over, Will. Pull over.' And then they started flipping."

"My dad said -- when my dad gained consciousness, he said that he saw my brother -- his seat belt snapped and he was ejected through the sunroof," she added.

The family has been tracking a "Help Find Will Norton" Facebook page and pursuing leads on his whereabouts.

Norton's aunt, Tracey, said the family received a tip that the teen was listed on a local hospital's emergency room roster -- but she's not sure where he is now.

"They transferred him, but we're not sure where he was transferred," the aunt said. "When he was transferred, he was alive. We don't know anything other than that."

The tornado that carved through the city of about 50,000 on Sunday is the deadliest to hit American soil since the National Weather Service began keeping records 61 years ago. The National Weather Service notes seven deadlier twisters, but says those took place "before the years of comprehensive damage surveys," so they may have been the result of multiple tornadoes.

But the National Weather Service does say that the Great Tri-State Tornado of 1925, which tore across southeast Missouri, southern Illinois, and southwest Indiana, killed 695 people -- "a record for a single tornado."

Last month, two fatal twisters struck Alabama. One hit Hackleburg and the town of Phil Campbell, killing 78 people, and another struck Tuscaloosa and Birmingham, killing 61.

"Everybody's going to know people who are dead," said Zach Tusinger, who said his aunt and uncle died in the Joplin tornado. "You could have probably dropped a nuclear bomb on the town and I don't think it would have done near as much damage as (the tornado) did."

With crews still sifting through rubble, the death toll could continue to climb.

"I think the more time that goes by, the more I feel sick about it," Hale said. "These people are cold and sick and stuck. As the days go on, and the death toll goes up, how many funerals are we going to go to?"

Joplin Mayor Mike Woolston said Monday night that his community hasn't given up.

"We hope that there are people alive. We have a number of apartment buildings, complexes that are almost completely flattened. So we anticipate finding more people, and hopefully we'll get there in time to find them alive," Woolston said.

Hale said Tuesday that she still hasn't slept since Sunday afternoon, when she didn't know whether her family across town had survived.

"I was hysterical. There's no words to describe not knowing if my family was alive," she said. "The only things left standing in their house was their bathtub and the toilet."

Her mother and grandparents did survive -- by huddling in the bathtub.

Neff, her fiance Zac Bronson and her toddler prayed, screamed and survived.

"We've had a tremendous support system. Our employers, friends and family have been more than helpful and we move on and rebuild. We just start another life. We started a new life," Bronson said.

The tornado chewed through a densely populated area of the city, damaging or destroying 2,000 buildings, eliminating a high school and making a direct hit on one of the two hospitals in the city.

Based on preliminary estimates, the twister carried winds between 190 and 198 mph, National Weather Service director Jack Hayes said.

More than 1,000 law enforcement officers from four states descended on Joplin to help with disaster response, said Collin Stosberg, a spokesman for the Missouri State Highway Patrol.

Also, 217 National Guard troops were on duty, said Maj. Tamara Spicer of the Missouri National Guard. About 30 military police are helping with security and checkpoints as people try to return to their home in areas that may not be safe, Spicer said.

The weather has been hampering wide-area airborne surveillance missions, she added.

The flood of aid from strangers and volunteers has helped ease the misery in Joplin.

"I've seen good-heartedness the past 24 hours like I've never seen in my life," Hale said. "As much help that has poured out from the nation, we need it. We need the help."

Woolston, the mayor, pledged not to let the tornado ruin his city.

"This is just not the type of community that's going to let a little F4 tornado kick our ass. So we will rebuild, and we will recover."

Online Public Information File

Viewers with disabilities can get assistance accessing this station's FCC Public Inspection File by contacting the station with the information listed below. Questions or concerns relating to the accessibility of the FCC's online public file system should be directed to the FCC at 888-225-5322, 888-835-5322 (TTY), or fccinfo@fcc.gov.